1. Field of the Invention
This invention relates to holograms and particularly a wide angle hologram device with an image that is reproduced from a near field illuminating source, and method of manufacturing same.
2. Description of Related Art
A hologram or hologram device is a device that includes a photosensitive film containing a recordation of an object. As is well-known, holograms are recorded with coherent light with a reference beam that strikes the photosensitive film directly and an object beam which strikes that object to be recorded and then strikes the film. The angles of these beams in relation to the plane of the film determines how the recorded image will be reproduced from the finished hologram, a process known as reconstruction. The image becomes visible when the hologram is illuminated by an appropriate light source from the same angle of either the reference beam or the object beam that was used during the recording process.
The angle between the reference beam and the object beam used in the recording determines the angle between the reconstructing light source and the reproduced image seen by the viewer that results in reproduction of the recorded image with maximum brightness and minimum distortion. This is often referred to as the “angle of reconstruction”. Holograms are often designed so as to be viewed by an observer directly facing the film of the hologram, with the recorded image being reproduced perpendicular to the film of the hologram. Therefore, often times the angle of reconstruction corresponds to an angle with respect to a perpendicular line to the hologram surface.
Generally, either the object beam or the reference beam used to create the hologram is arranged at an angle of approximately 45 degrees with respect to a perpendicular line to the hologram surface, since this geometry is easy to record. In this case, a reconstruction light source positioned at the same angle of approximately 45 degrees with respect to a perpendicular line to the hologram surface is used for reconstruction, since this geometry results in reproduction of the recorded image with maximum brightness and minimum distortion.
Also well known, images from conventional holograms achieve maximum sharpness when reconstructed from collimated light, ideally from a point source. Therefore the light source is positioned at a distance from the hologram which causes the light to be naturally collimated.
FIG. 1A shows reconstruction of a conventional hologram with a distance the light source angled approximately 45 degrees from the hologram. Basically, the light from a light source is well above (or below) the hologram and the viewer.
In many applications, however, it may be advantageous to have a wide angle between the reconstruction beam and a perpendicular line to the film, from a light source positioned close to the hologram, as shown in FIG. 1B. One such application, for example, may be a thin wall display for advertising.
Others have described so-called edge light holograms. U.S. Pat. No. 4,643,515 to Upatnieks describes a technique that provides a monochromatic virtual image at infinity behind an edge-lit hologram. However a compact display is not possible due to reconstruction involving a laser positioned at a distance from the hologram.
A compact system is described in U.S. Pat. No. 5,121,229 to Benton et al., which requires a three-step process to produce a master used for duplication. Furthermore, the recording of Benton is performed using a mask with a slit which sacrifices vertical parallax. Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight. For example, a hologram with horizontal parallax means that various horizontal perspectives of the image have been recorded, thereby allowing an observer to view these perspectives as the observer's position changes in a horizontal direction. Sacrificing vertical parallax, as occurs in Benton, means that only the top or the bottom of the image is able to be recorded in the hologram. As a result, an observer will only be able to view one vertical perspective of the image (i.e., either the top or the bottom, but not both), depending on which vertical perspective was recorded. This is the case regardless of whether the observer is viewing the hologram from the top or the bottom of the reconstructed image.
U.S. Pat. No. 4,512,624 to Nicholson describes a two step process for producing a transmission hologram that is reconstructed with a diverging beam of white light. However, this technique also uses a mask during recording and requires a light source position at a distance from the hologram in practical applications. As such, the hologram device of Nicholson does not reconstruct its image when illuminated with a near field source from a wide angle with respect to a perpendicular line to the film of the hologram device. In addition, the Nicholson device also sacrifices some vertical parallax.
U.S. Pat. No. 6,614,565 to Klug et al. describes a one-step process requiring extensive hardware (including computers) and sophisticated image manipulation, and is comparatively expensive.
Thus, it is desirable to provide a low cost, two-step method for creating a hologram device which reconstructs its image when illuminated with a near field source from a wide angle with respect to a perpendicular line to the film of the hologram device.